One of the optical performances required for many optical materials made of plastic is a refractive index. Particularly for an optical material with a high refractive index, many episulfide compounds capable of providing an optical material with a refractive index of not less than 1.7 have been discovered (Patent Documents 1 to 4).
As a polymerization catalyst for a composition containing an episulfide compound, onium salts such as quaternary ammonium salts, quaternary phosphonium salts, tertiary sulfonium salts and secondary iodonium salts, which have a halogen in an anionic part, have been disclosed (Patent Document 5). However, in recent years, from a perspective of environmental load reduction, a halogen-free material has been desired.
On the other hand, as a curing accelerator for an epoxy resin, tetraalkylphosphonium dialkylphosphates have been disclosed (Patent Document 6). However, it has not been described that these can be used as a polymerization catalyst for an episulfide compound.